What is a Gamma Ray Burst? Its types and causes…

 …continue from Part 1 – its history and Part 2 – its location

Now we know that gamma ray bursts (GRBs) are far far away and releasing extremely HUGE amount of energy. They are by far the brightest and most energetic phenomena in the known universe, second only to the Big Bang itself.

What could possibly cause them?

In fact, we are not really sure…

But we have good guesses…

Before we go into what could possibly cause a gamma rays burst, let us first look at the types of GRBs because different types of bursts are created fundamentally by different processes. In general, GRBs are separated into two classes – short-duration bursts and long-duration bursts.

The short-duration bursts last less than 2 seconds and range anywhere from a few milliseconds to 2 seconds with an average duration time of about 0.3 seconds. The long-duration bursts are those last more than 2 seconds and up to few minutes. The average time for long-duration burst is about 30 seconds. 

A gamma-ray burst heralds the birth of a black hole – an artist’s concept. Taken from Science@NASA.

Astronomers now are quite sure what causes the long-duration GRBs. The explanation was first put forth by Dr. Stan Woosley of University of California in his “collapsar” model. At the end of the life of a very very massive star, about 50 to 100 times more massive than our Sun, its core collapsed and formed a black hole. At the same time, the star’s outer layers exploded energetically and it funnel the energy out through the two poles and form a GRB. The energy outputs of these so-called “hypernovae” are much greater (by about 100 times) than a normal supernova.

Although we are now fairly confident of what drives the long-duration GRBs, the short-duration ones still remains a mystery. The durations of the bursts are too short to be supernovae, and the energies involved do not add up to an exploding star. It was found that the short-duration ones release more energy relative to the longer ones.

Our best guess is that short duration GRBs are caused by the extremely violent collisions of two compact objects, such as colliding of two neutron stars or merging of a neutron star and a black hole. When these very compact objects merged, there are going to release a huge amount of energy and ended up as a new black hole (in the case of two colliding neutron stars) or a bigger black hole (in the case of a neutron star and a black hole merging).


An artist’s concept of a neutron star-neutron star collision. Taken from Science@NASA.

We never “see” these kinds of collisions in action, so how do we know that this explanation is true?

Before the two objects collide, they would orbit each other as a binary system. At the same time, they lose their rotational energy to gravitational energy, i.e. their orbit decays, and they spiral in toward each other. As they spiral in, these dense objects intense gravitation field will cause ripples in the fabric of spacetime and can be detected as gravitational waves. The frequency of these waves would ramp up in a characteristic pattern as the objects get closer and closer to each other until… bang! they collided in a fiery explosion, and release burst of gamma rays.

 

<= Gravitational wave – “ripples” in the curvature of space-time – generated by the motion of massive objects. Credit: K. Thorne (Caltech) , T. Carnahan (NASA GSFC)

Astronomers are trying to detect these waves now. The Laser Interferometer Gravitational-wave Observatory (LIGO) and future Laser Interferometer Space Antenna (LISA) hopefully will be able to detect these gravitational waves and help verifying the model.

These are currently some of our best guesses. It may also be that GRBs originated from something that we have not thought of yet.

Next, we are going to look at the space telescopes with gamma rays eyes.  …to be continued…

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~ by thChieh on November 7, 2008.

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